1. Field of the Invention
The present invention relates to a dot-matrix fluorescent display device. More particularly, it relates to a dot-matrix fluorescent display device employing a fluorescent display tube.
2. Background Art
A dot-matrix fluorescent display device employing a fluorescent display tube is often used for a video or audio device to indicate by a selected lit numeral or character a received frequency, a selected mode or the like. The general operation of the fluorescent display tube is hereafter described with reference to FIG. 1. In the tube, a filament 1 is heated by electric power from an AC power supply 2 to thereby emit thermoelectrons. The electrons are accelerated and diffused by a grid g.sub.i to which a grid drive voltage VG.sub.i is applied. After passing through the grid g.sub.i, the electrons collide against a plurality of anodes A.sub.ij to which a separate anode drive voltage VA.sub.ij is applied to each of the plurality of anodes A.sub.11 -A.sub.mn. As a result, a light emitter (not shown in the drawing) provided on the surface of each anode A.sub.ij emits light. If the fluorescent display tube has 5.times.7 (35) image elements, 5.times.7 (35) anodes A.sub.ij are disposed in a matrix to form an anode group AG.sub.i, as shown in FIG. 2. Anode lead wires 1.sub.11 to 1.sub.75 are connected to the anodes A.sub.11 -A.sub.75. The grid g.sub.i is provided in opposition to every matrix-disposed anode group AG.sub.i, as shown in FIG. 4. Multiple anode groups AG.sub.i provide multiple displayed characters.
FIG. 3 shows an enlarged sectional view of the anode A.sub.ij included in the fluorescent display tube. The light emitter 3 is provided on the surface of the anode A.sub.ij. The anode lead wire 1.sub.ij is connected to the anode A.sub.ij.
FIG. 4 shows a schematic view of a conventional dot-matrix fluorescent display device employing such a fluorescent display tube as described above. In the device, each of a number n of anode groups AG.sub.1 -AG.sub.n corresponds to one character. The n anode groups AG.sub.1 -AG.sub.n correspond to a line of characters. In each anode group AG.sub.i, 5.times.7 (35) anodes A.sub.ij are disposed in a matrix, as shown in FIG. 2. The anode lead wire 1.sub.ij for the anode A.sub.ij of the anode group AG.sub.i is connected to the anode lead wire 1.sub.ij for the anode A.sub.ij of the adjacent anode group AG.sub.i+1. An anode drive voltage VA.sub.ij is applied through the anode lead wire 1.sub.ij to the anode A.sub.ij with its attached light emitter 3, to cause it to emit light. It is probable that the anode drive voltage VA.sub.ij is applied to a plurality of similarly situated anodes A.sub.ij simultaneously. Grids g.sub.1 -g.sub.n are independently provided for the anode groups AG.sub.1 -AG.sub.n. Grid drive voltages VG.sub.1 -VG.sub.n are sequentially applied to the grids g.sub.1 -g.sub.n in a time division multiplex manner.
FIG. 5 shows a timing diagram of examples of the grid drive voltages VG.sub.1 -VG.sub.n and the anode drive voltage VA.sub.ij for the dot-matrix fluorescent display device shown in FIG. 4. Since the grid drive voltages VG.sub.1 -VG.sub.n are applied to the grids g.sub.1 -g.sub.n in time division multiplex and the anode drive voltage VA.sub.ij is applied to the anodes A.sub.ij of the anode groups AG.sub.1 -AG.sub.n, the light emitter 3 emits light only if it is located on the anode A.sub.ij of the anode group AG.sub.i for which the grid drive voltage VG.sub.1 and the anode drive voltage VA.sub.ij are simultaneously applied. For the same reason, the light emitter 3 provided on the anode A.sub.ij of the anode group AG.sub.n later emits light as well for a different combination of grid drive voltage VG.sub.n and anode drive voltage VA.sub.ij.
The above-mentioned conventional dot-matrix fluorescent display device needs a number of anode lead wires 1.sub.ij, which number corresponds to that of the number of anodes A.sub.ij which constitute each of the anode groups AG.sub.i. If each anode group AG.sub.i is composed of 5.times.7 (35) anodes A.sub.ij in a matrix group, 35 anode lead wires 1.sub.ij are needed. However, in reality, anode drive voltage is simultaneously applied to several anodes A.sub.ij through the corresponding anode lead wires 1.sub.ij. When only a predetermined image such as "PLAY" and "FF" is to be indicated by the dot-matrix fluorescent display device for a video or audio device, anode drive voltages are always simultaneously applied to a plurality of anodes A.sub.ij in predetermined anode groups AG.sub.i through the corresponding anode lead wires 1.sub.ij. For that reason, the number of all the anode lead wires 1.sub.ij of the device is so large that the device is very complicated.